Ammonia generator, vehicle and method for generating ammonia

ABSTRACT

The invention relates to an ammonia generator ( 2, 66, 68 ), in particular for mobile applications, a vehicle ( 62 ) with such an ammonia generator ( 2, 66, 68 ) and a method for generating ammonia from a waste gas of a combustion process. It is proposed that the waste gas originates from a free-piston engine ( 2 ) which is operated with a fuel.

The invention relates to an ammonia generator, in particular for mobile applications, as generically defined by the preamble to claim 1; a vehicle having such an ammonia generator as generically defined by the preamble to claim 5; and a method for generating ammonia as generically defined by the preamble to claim 11.

PRIOR ART

In a vehicle of the type defined at the outset, for exhaust gas treatment of oxygen-rich gasoline and diesel exhaust gases from an internal combustion engine of the vehicle for eliminating nitrogen oxides, it is already known from German Patent Disclosure DE 101 45 808 A1 of the present Applicant to employ what is known as the SCR (Selective Catalytic Reduction) process, in which the nitrogen oxides (NO_(x)) in the exhaust gas are reduced in an SCR catalytic converter, after prior delivery of ammonia (NH₃) to nitrogen (N₂) with high selectivity. The ammonia needed for the SCR process is generated there, in an ammonia unit carried along on board the vehicle, for instance by the Haber-Bosch process, from nitrogen and hydrogen at a pressure of more than 100 bar.

It is also already known, from German Patent Disclosure DE 199 22 960 A1, to generate the ammonia required for reducing nitrogen oxides by the SCR process in an exhaust gas cleaning system of an internal combustion engine on board, in an ammonia generator catalytic converter from combustion exhaust systems of the internal combustion engine, whose cylinders are for that purpose acted upon at least intermittently or partially with a rich fuel-air mixture, so that the exhaust gas emitted by the internal combustion engine contains not only uncombusted hydrocarbons but also hydrogen and a certain quantity of nitrogen oxides. From these latter two ingredients, ammonia is then generated in the ammonia generator catalytic converter. However, generating ammonia from exhaust gases of the internal combustion engine has the disadvantage that the internal combustion engine cannot be adjusted optimally in terms of either its fuel consumption or the driving qualities of the vehicle.

ADVANTAGES OF THE INVENTION

The ammonia generator of the invention having the characteristics recited in claim 1, the vehicle of the invention having the characteristics recited in claim 5, and the method of the invention having the characteristics recited in claim 11 by comparison offer the advantages that the efficiency of ammonia generation, compared to generation from exhaust gases of an internal combustion engine serving as a drive motor for the vehicle, can be improved; that because of the decoupling of the ammonia generation and the operation of the internal combustion engine as a vehicle driving motor, an impairment in the driving qualities of the vehicle and an increase in fuel consumption can be avoided; and that the free-piston engine, which forms part of the ammonia generator, can furthermore be used in the vehicle as an “auxiliary power unit” or APU, preferably for generating hydraulically usable energy, for instance for a high-pressure injection system of the vehicle.

A preferred feature of the invention provides that the exhaust gas of the free-piston motor, for synthesizing the ammonia, is conducted through a catalytic converter, preferably a noble-metal/ammonia catalytic converter; beforehand, fuel is optionally added to the exhaust gas, in order to increase its content of uncombusted hydrocarbons, from which hydrogen is catalytically split off in the catalytic converter and used for the catalytic synthesis of ammonia. Moreover, with the aid of the uncombusted hydrocarbons, oxygen that is a hindrance to the ammonia synthesis taking place in the catalytic converter is reactively removed.

In a further preferred feature of the invention, the ammonia catalytic converter communicates with an exhaust system of the internal combustion engine that serves to drive the vehicle, specifically upstream of an SCR catalytic converter, in which the nitrogen oxides contained in the exhaust gas of the internal combustion engine and of the free-piston engine are reduced catalytically to nitrogen with high selectivity, using the previously synthesized ammonia.

As already indicated, the free-piston engine can not only be used as part of an ammonia generator for obtaining ammonia but can simultaneously and advantageously also be used as an “auxiliary power unit” (APU) of the vehicle, for instance to increase the pressure of a fuel injected into the internal combustion engine in order to supply the required high-pressure oil to a power steering system of the vehicle, or for hydraulic brake boosting in the case of heavy utility vehicles.

To increase the fuel pressure with the aid of the free-piston engine, the fuel is expediently delivered directly to a work cylinder of the free-piston engine, in a first pressure elevation stage, and then with the aid of a downstream pressure booster increased with further boosting stages to the desired pressure level of between 300 and 2000 bar.

DRAWINGS

The invention will be described in further detail below in terms of an exemplary embodiment in conjunction with the associated drawings. Shown are:

FIG. 1, a schematic illustration of the functional principle of a free-piston engine;

FIG. 2, a schematic illustration of a motor vehicle having an internal combustion engine and a free-piston engine, the latter forming part of an ammonia generator and a pressure generator.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

The free-piston engine 2 shown in FIG. 1, operating by the two-stroke process, has a cylinder 4 in a manner known per se, in which a free piston 8, mounted on the end of a piston rod 6, is freely movable. The free piston 8 subdivides a cylinder chamber 10, enclosed by the cylinder 4, into two chambers: a fresh-air chamber 12, on the side toward the piston rod 6, and a combustion chamber 16, on the opposite end of the free piston 8; on one face end the combustion chamber is provided is provided with an injection nozzle 14. When the free piston 8 is located in the vicinity of its “bottom” dead center, shown in FIG. 1, the two chambers 12 and 16 communicate with the ambient air via an air inlet 18 and with an exhaust gas line 22 via an exhaust 20, and they communicate with one another on the two-stroke motor principle, for instance through lateral air slits 24 (only one of which is shown).

A further piston 26 with a smaller cross section is fixedly mounted on the piston rod 6 and is movable in a compression or restoration cylinder 28 that adjoins the cylinder 4 on the side of the fresh-air chamber 12. The compression or restoration cylinder 28 is subdivided by the piston 26 into a compression chamber 30 and a restoration chamber 32, of which the first can be made to communicate with a compression reservoir 38 via two bores 34, 36. The bore 36 adjacent to the face end of the compression chamber 30 has a small diameter, and it communicates with the compression reservoir 38 via a check valve 40 and a regulating valve 42, while the other bore 34 has a larger diameter and communicates directly with the compression reservoir 38, but in the vicinity of the “bottom” dead center of the free piston 8 shown in FIG. 1, it is closed by the piston 26.

From the compression or restoration cylinder 28, the piston rod 6 extends onward into a work cylinder 44, which forms part of a hydraulic circuit 46, through which an incompressible hydraulic fluid flows and in which the pressure of the hydraulic fluid is to be increased by means of the free-piston engine 2. The hydraulic circuit 46 includes a high-pressure working reservoir 48 on the high-pressure side of the work cylinder 44 and a low-pressure working reservoir 50 on the low-pressure side of the work cylinder 44; these reservoirs each communicate in such a way, via a respective check valve 52 and 54, with a work chamber 56 enclosed by the work cylinder 44 that the hydraulic fluid is aspirated from the low-pressure working reservoir 50 into the work chamber 56 and from there is expelled under pressure into the high-pressure working reservoir 48. The hydraulic circuit 46 further includes a consumer, communicating with the high-pressure working reservoir 48 or the high-pressure side of the work cylinder 44 via a high-pressure line 58, and also includes a return line 60 from the consumer to the low-pressure working reservoir 50.

In operation of the free-piston engine 2, in the outset position shown in FIG. 1 at “bottom” dead center of the free piston 8, a fluid under pressure, stored in the compression reservoir 38, flows slowly via the regulating valve 42 and the bore 36 into the compression chamber 30, whereupon the piston 26 and thus also the piston rod 6 and the free piston 8, at the onset of an “intake and compression stroke”, move gradually in the direction of the combustion chamber 16. In this process, fresh air is first aspirated through the air inlet 18 into the fresh-air chamber 12, and exhaust gas from the combustion chamber 16 is expelled through the exhaust 20 into the exhaust gas line 22. As soon as the free piston 8 closes the air inlet 18, the outlets toward the combustion chamber of the air slits 24, and the exhaust 20, the piston 26 opens the second bore 34, and as a result the compression reservoir 38 is abruptly evacuated into the compression chamber 30. As a result, the piston 26 and thus also the free piston 8 are accelerated in the direction of the injection nozzle 14 and rapidly compresses the air in the combustion chamber 16, causing the air to heat up markedly. Shortly before the free piston 8 reaches its “top” dead center in the vicinity of the injection nozzle 14, fuel is injected through this nozzle into the combustion chamber 16. The fuel is atomized, partly evaporated, and ignites, causing the free piston 8, during its ensuing “working stroke” moves back in the direction of the “bottom” dead center (FIG. 1). Shortly before “bottom” dead center is reached, compressed air from the fresh-air chamber 12 is forced via the air slits 24 into the combustion chamber 16 and flushes out this chamber, and the exhaust gas escapes into the exhaust gas line 22 through the exhaust 20.

In the region of the compression cylinder 28, during the working stroke, the compression reservoir 38 is first re-charged via both bores 34, 36 and then via the bore 36 and the check valve 40, while in the region of the work cylinder 44, hydraulic fluid is positively displaced by the piston rod 6 out of the work chamber 56 into the high-pressure working reservoir 48, this hydraulic fluid having been aspirated into the work chamber 56 from the low-pressure working reservoir 50 during the preceding “intake and compression stroke”.

It has been found that by a suitable choice of the operating point of the free-piston engine 2, among other provisions by means of suitable adjustment of the fuel-air ratio in the combustion chamber 16, the exhaust gas produced in combustion can be utilized with good efficiency to obtain ammonia, since it is possible for free-piston engines to be optimized in a very targeted way at one or only a few operating points with regard to the generation of ammonia.

FIG. 2 schematically shows a motor vehicle 62, with a driving motor embodied as an internal combustion engine 64; in this vehicle, the free-piston engine 2 serves on the one hand as a preliminary stage for generating ammonia as a reducing agent for the nitrogen oxides contained in the exhaust gas of the internal combustion engine 64, and thus serves the purpose of reducing emissions, and on the other is used as an “auxiliary power unit” (APU), in order to furnish the pressure required for high-pressure fuel injection into the internal combustion engine 64.

For generating ammonia, the exhaust gas from the combustion chamber 16 of the free-piston engine 2 is delivered through the exhaust gas line 22 to an oxidation catalytic converter embodied as a noble-metal/ammonia catalytic converter 66. In the noble-metal/ammonia catalytic converter 66, some of the hydrocarbons contained in the exhaust gas are catalytically broken down, splitting off hydrogen, and the hydrogen is used for the catalytic synthesis of ammonia from molecules contained in the exhaust gas that contain nitrogen, such as atmospheric nitrogen or nitrogen oxides. To increase the content of uncombusted hydrocarbons in the exhaust gas that are required for synthesizing ammonia, a postinjection of fuel into the exhaust gas can also be done upstream of the catalytic converter 66, at 68.

The outlet of the ammonia catalytic converter 66 communicates with an exhaust system 70 of the internal combustion engine 64; besides an oxidation catalytic converter 72 and optionally a particle filter (not shown), the exhaust system includes an SCR catalytic converter 74, in which the nitrogen oxides, contained in the exhaust gas from the internal combustion engine 64 and in the exhaust gas of the free-piston engine 2, are reduced with high selectivity to nitrogen, in order to reduce the nitrogen oxide emissions from the motor vehicle 62, with the aid of the ammonia synthesized in the catalytic converter 66 and delivered to the exhaust system 70 upstream of the SCR catalytic converter 74.

To furnish the fuel pressure required for the high-pressure fuel injection into the internal combustion engine 64, the work cylinder 44 of the free-piston engine 2 is supplied with fuel as its hydraulic fluid, the pressure of the fuel being increased in the work cylinder 44. To that end, the low-pressure side of the work cylinder 44 communicates directly with the fuel tank 76 of the motor vehicle 62, and this tank contains the fuel both for operating the internal combustion engine 64 and for operating the free-piston engine 2 and corresponds to the low-pressure working reservoir 50 in FIG. 1. The high-pressure side of the work cylinder 44 communicates with a pressure booster 78 that includes a plurality of pressure booster stages (not shown), with the aid of which the fuel pressure, increased in the work cylinder 44, can be boosted to a desired level of between 300 and 2000 bar, before the fuel from the pressure booster 78 is delivered to a fuel injection system 80 of the internal combustion engine 64. 

1-10. (canceled)
 11. A motor vehicle comprising: an internal combustion engine and a SCR catalytic converter in an exhaust system of the internal combustion engine; an ammonia generator having devices for generating ammonia from an exhaust gas produced in a combustion process, the devices including a free-piston engine operated with fuel, wherein the ammonia is generated from exhaust gas of the free piston engine, and wherein the free-piston engine increases pressure of fuel being injected into the internal combustion engine.
 12. The vehicle according to claim 11, wherein that the ammonia generator includes a catalytic converter through which the exhaust gas of the free-piston engine flows.
 13. The vehicle according to claim 12, wherein that the catalytic converter is a noble-metal/ammonia catalytic converter.
 14. The according to claim 12, wherein the ammonia generator includes a fuel injecting device for injecting fuel into the exhaust gas of the free-piston motor, before the exhaust gas passes through the catalytic converter.
 15. The according to claim 13, wherein the ammonia generator includes a fuel injecting device for injecting fuel into the exhaust gas of the free-piston motor, before the exhaust gas passes through the catalytic converter.
 16. The vehicle according to claim 12, wherein the catalytic converter of the ammonia generator through which the exhaust gas of the free-piston engine flows, is in communication with the exhaust system of the internal combustion engine upstream of the SCR catalytic converter.
 17. The vehicle according to claim 13, wherein the catalytic converter of the ammonia generator through which the exhaust gas of the free-piston engine flows, is in communication with the exhaust system of the internal combustion engine upstream of the SCR catalytic converter.
 18. The vehicle according to claim 14, wherein the catalytic converter of the ammonia generator through which the exhaust gas of the free-piston engine flows, is in communication with the exhaust system of the internal combustion engine upstream of the SCR catalytic converter.
 19. The vehicle according to claim 15, wherein the catalytic converter of the ammonia generator through which the exhaust gas of the free-piston engine flows, is in communication with the exhaust system of the internal combustion engine upstream of the SCR catalytic converter.
 20. The vehicle according to claim 11, wherein the free-piston engine forms an “auxiliary power unit” (APU) of the vehicle.
 21. The vehicle according to claim 12, wherein the free-piston engine forms an “auxiliary power unit” (APU) of the vehicle.
 22. The vehicle according to claim 13, wherein the free-piston engine forms an “auxiliary power unit” (APU) of the vehicle.
 23. The vehicle according to claim 11, wherein a work cylinder of the free-piston engine can be subjected directly to the fuel.
 24. The vehicle according to claim 12, wherein a work cylinder of the free-piston engine can be subjected directly to the fuel.
 25. The vehicle according to claim 13, wherein a work cylinder of the free-piston engine can be subjected directly to the fuel.
 26. The vehicle according to claim 11, wherein a pressure booster is disposed between the work cylinder of the free-piston engine and a fuel injection system of the internal combustion engine.
 27. The vehicle according to claim 12, wherein a pressure booster is disposed between the work cylinder of the free-piston engine and a fuel injection system of the internal combustion engine.
 28. The vehicle according to claim 13, wherein a pressure booster is disposed between the work cylinder of the free-piston engine and a fuel injection system of the internal combustion engine.
 29. An ammonia generator for a vehicle having an internal combustion engine and a SCR catalytic converter in an exhaust system of the internal combustion engine, comprising devices for generating ammonia from an exhaust gas produced in a combustion process, the devices including a free-piston engine operated with fuel, wherein the ammonia is generated from exhaust gas of the free piston engine, and wherein the free-piston engine increases pressure of fuel being injected into the internal combustion engine.
 30. A method for generating ammonia from an exhaust gas of a combustion process, the method used in a vehicle having an internal combustion engine and an SCR catalytic converter in an exhaust system of the internal combustion engine, comprising the steps of: producing the exhaust gas in a combustion process in a free-piston engine operated with fuel; and increasing the pressure of a fuel injected into the internal combustion engine by means of the free-piston engine. 